1. Field of the Invention
The present invention relates to an apparatus for growing a mixed compound semiconductor comprising at least three elements by vapor phase epitaxy. More particularly, the present invention relates to an apparatus for growing a mixed compound semiconductor having a uniform mixing ratio and a growth method using the same.
The mixed compound semiconductor comprises at least two binary compound semiconductors. A different energy is necessary for forming each binary compound semiconductor. When a plurality of source gases, each comprising an element gas are mixed and introduced into a reaction chamber, a grown mixed compound semiconductor layer on a substrate has non-uniform mixing ratios (x-values) depending on a position of the substrate surface. The present invention relates to an apparatus which is used for growing a compound semiconductor layer having uniform x-values throughout the entire semiconductor layer grown on the substrate surface.
2. Description of the Related Art
A prior art method of growing a mixed compound semiconductor utilizes an apparatus schematically shown in FIG. 1. An exemplary ternary semiconductor Hg.sub.1-x Cd.sub.x Te of a group II-VI compound semiconductor is used for explaining the prior art method using the apparatus of FIG. 1. The mixed compound semiconductor Hg.sub.1-x Cd.sub.x Te has a small energy bandgap characteristic and is known as a detector material for infrared rays. The mixing ratio x included in the above expression Hg.sub.1-x Cd.sub.x Te is defined as a ratio of a binary compound semiconductor CdTe to a ternary compound semiconductor HgCdTe and plays an important role in determining the most sensitive wavelength of infrared rays during detection.
A mercury (Hg) bubbler 1 supplies mercury vapor contained in a bubbling hydrogen gas. A diisopropyltelluride (abbreviated hereinafter as DIP-Te) bubbler 2 supplies DIP-Te vapor contained in a bubbling hydrogen gas. A dimethylcadmium (abbreviated hereinafter as DM-Cd) bubbler 3 supplies DM-Cd vapor contained in a bubbling hydrogen gas. The above three source gases are introduced into a reactor chamber 4 in which a substrate stage 5 is arranged. A substrate 6 of, for example, gallium arsenide (GaAs) is disposed on the stage 5 which is heated during growth of the compound semiconductor by an RF coil 7 arranged outside the reactor chamber 4. DIP-Te and DM-Cd source gases are decomposed into element gases, and Te, Cd and Hg molecules react with each other forming a binary semiconductor CdTe and HgTe around the heated substrate and deposit on the substrate 6 forming an epitaxial Hg.sub.1-x Cd.sub.x Te layer. The method falls under the category called Metal Organic Chemical Vapor Deposition (MOCVD).
The above method includes a problem that the binary compound semiconductor CdTe is first formed on the upstream side of the mixed source gas flow in the reactor chamber 4 and the binary compound semiconductor HgTe is later formed on the downstream side, since the energy necessary for forming CdTe is smaller than that for forming HgTe. Therefore, the x-value of the grown ternary compound semiconductor Hg.sub.1-x Cd.sub.x Te is not uniform on the substrate surface. This is shown in an exaggerated manner by the solid curves 21 and 22 in FIG. 2. The abscissa represents an arbitrary relative distance from an input end of the reactor chamber 4 and the ordinate represents a relative quantity of formed binary semiconductors. Curve 21 denotes that the binary semiconductor CdTe alone is first formed on the upstream side of the reactor chamber and curve 22 denotes that the binary semiconductor HgTe is formed on the downstream side of the reactor chamber. As a result, the grown ternary semiconductor Hg.sub.1-x Cd.sub.x Te on the substrate has a composition such that the x-value thereof gradually decreases from 1 (upstream side) to 0 (downstream side) in an extreme case. The larger the distance D between peak positions of the two curves, the more the change in x-value on the surface of the grown ternary compound layer Hg.sub.1-x Cd.sub.x Te.
As explained above, when at least three source gases are mixed and thereafter introduced into a reaction chamber for growing a mixed compound semiconductor, a binary compound semiconductor having a smaller formation energy is formed on the upstream side of the reaction chamber, and a binary compound semiconductor having a larger formation energy is formed on the downstream side thereof.
In order to improve uniformity of the grown semiconductor, Japanese Unexamined Patent Publication SHO 63-318733 discloses a vertical reactor type MOCVD apparatus in which a preliminary heating plate having a plurality of throughholes which is heated during the growth is disposed on the upstream side of the reactor. Japanese Unexamined Patent Publication HEI 1-201926 discloses a horizontal reactor type MOCVD apparatus, in which heating means is provided in the reactor. However, according to the above two references, since source gases are heated almost uniformly, the difference in formation energies cannot be solved.